Interferometers have been known and used for a long time. Interferometry is a widely used method for measuring surface profiles (often to nano-meter resolutions) and other physical properties of materials, gases and liquids. There are many types of interferometers, characterized by their optical designs and layouts. Some classical types are Twyman-Green, Fizeau, Michaelson, Mach-Zender, and Fabry-Perot. Each of these interferometer types produces interference patterns, called interferograms. These interferograms can be used to analyze characteristics of an object under test.
Interferograms are generated by the interference of a test wavefront and a reference wavefront. The test and reference wavefronts typically originate from a common source and are obtained by splitting the originating wavefront. The test wavefront then obtains information about the test object by interacting with the object under test (typically by reflecting off of, or transmitting through a test object). Similarly, the reference wavefront obtains its “reference” information by interacting with a “known” reference object, such as a super polished flat glass plate. Superimposing or interfering these two wavefronts (i.e. on a flat screen, or an image sensor such as a CCD) produces an interferogram.
Interferometers require coherent superposition of a “test beam” (of light) with a “reference beam” (“beam” and “wavefront” used interchangeably herein, with a “wavefront” being understood as propagating along the optical axis and sweeping out a volume that defines the light beam) resulting in the formation of an interferogram in the overlapping region of the two beams. This interferogram data can then be captured using various types of detectors, such as a camera, for analysis.
The spatial distribution of intensity levels within the interference pattern relates to differences in the phase of the test and reference wavefronts. Note that the reference wavefront is acted on by a known “measurement standard,” such as an optical “reference” surface, and the test wavefront is acted on by the unknown object under test. Measuring the difference between the two wavefronts allows the test wavefront to be determined. In other words, the process is akin to comparing the “unknown” test wavefront to a “known” standard, the reference wavefront.
A single interferogram is usually insufficient to obtain the accuracy required for many applications. A variety of methods have been developed to acquire multiple phase-shifted interferograms as a means to increase accuracy and resolution of the measurement. Phase-shifting techniques require altering the phase between the two interfering wavefronts by introducing controlled phase delays between the test and reference beams. These added phase-shifts supply additional information that can be used to compute the test wavefront significantly more accurately. Almost all current techniques of phase shifting use sequential or “temporal” methods to introduce phase differences while multiple interferograms are acquired serially in time. However, in practice, these temporal methods cannot be used effectively in the presence of relatively fast changing environmental conditions (such as vibrations, air turbulences, etc), or when the object under test cannot be stabilized (i.e. vibrating), or when the object under test is in motion.
White light scanning interferometers (“WLSI”) belong to a special group of optical measuring instruments that use the interference fringes of an interferogram created by a source with short coherence length. The interference fringes are formed from the interference of light reflected from a test object and a reference element, and the respective reflected lights are superimposed on an image of the object indicating location of those parts of the object's surface that are of the same distance to the reference. In this manner, a white light scanning interferometer can measure surface topography with very high accuracy, commonly exceeding 1 nm. The nature of the measurement requires that the object must be translated along the optical axis of the interferometer so that locations of all points on the surface of the object are compared with the reference.
The white light scanning interferometer is a well established measuring instrument. Interferometers based on this principle are most widely used in the inspection of parts in manufacturing, where they provide information about surface topography of the parts. White light scanning interferometry is a well-suited interferometry method for various reflecting and scattering surfaces, for different shapes and for different materials.
FIG. 1 shows a schematic diagram of a typical configuration of a white light scanning interferometer, as known in the prior art. The instrument includes three functional parts: measuring head 5, scanner 70 and base 80. The measuring head 5 includes light source 10 used to illuminate the measured object 30. The measuring head 5 further includes a beam splitter 20 that is a part of the interferometer system and is used to produce test and reference beams, a reference mirror 40, imaging optics 60, and an image detector 50. In this configuration, the beam splitter 20 is also used to combine light reflected by an object 30 and a reference mirror 40 so that the two beams leaving the beam splitter overlap and are directed towards image detector 50. Imaging optics 60 is used to create a sharp image of the reference mirror on the detector. Typically, a pixelated image detector such as a monochromatic CCD camera is used as the image detector 50 to capture the image and transfer it to a computer.
In operation, the white light scanning interferometer is used to measure topography of a top surface of the object 30. Light from the source 10 is split into a test and a reference beam in the beam splitter 20. They recombine after reflecting from the top surface of the object 30 and reference mirror 40 respectively. The imaging optics creates a sharp image of the reference mirror on the camera 50. In the process, those parts of the object 30 that are in focus for the current position of the scanner are also sharply imaged on the camera 50. The light from the test and reference beams interfere with each other creating a set of interference fringes superimposed on the image of the object 30 on the camera 50. These fringes form only for those parts of the object 30 for which the distance to the beam-splitter 20 differs from the distance between the beam-splitter 20 and the reference mirror 40 by no more than the coherence length of the source. Contrast (visibility) of the interference fringes varies as a function of the distance difference and is highest for those parts of the object for which the distance to the beam-splitter is equal to the distance between the beam-splitter and the reference mirror. Using contrast of the interference fringes as an indicator, the distance difference between measured surface and the reference mirror can be measured with accuracy better then 1 nm. Typically, a short coherence length of the illumination source is required (usually several micrometers) which can be adjusted to some degree by properly filtering light from the source.
For a given position of the measuring head only those parts of the object 30 can be measured for which interference fringes exist. Thus, in order to measure all points of the object the measuring head must be translated by a distance covering the entire range of heights for all the points creating the top surface of the object 30. In practice, by continuously scanning the optical head 5 with respect to the object 30, starting from the lowest point of the surface and continuing through the highest point on the surface, a complete profile of surface topography can be measured.
The scanning procedure, being an integral part of the measurement, is also a source of major limitations in white light interferometry. The sampling interval in scanning is limited by the period of the interference fringes and is usually fixed to a certain fraction of light wavelength. As a consequence, a large number of images are required for even moderate scans. This makes the measurement with a WLSI a slow process, with scanning speeds typically on the order of 5-7 μm/s. Because the measurements require an extended time to complete, the method is sensitive to mechanical vibrations and often requires that the instrument be placed on a vibration isolation support. Therefore, sensitivity to vibrations almost completely precludes use of the white light interferometers in production environments and limits their use to measurement laboratories.
Recently some of the limitations mentioned above were reduced to some degree. It has been shown that the speed of scanning could be increased through the use of different algorithms for data processing that allow sub-sampling of the interference pattern and allow for increasing the sampling interval. This approach permits scanning speed on the order of 30 μm/s, but at an expense of a loss in measurement accuracy. Other recently developed techniques have shown that by precise synchronization of scanning speed and sampling interval it is possible to increase the scanning speed to about 100 μm/s. This method, however, requires more expensive hardware, involves precise calibration of the instrument, and causes a further decrease in measurement accuracy.
Thus, a method that would be capable of further increasing the scanning speed of white light interferometers and/or that would make them less sensitive to vibrations would be very desirable to overcome some of the most limiting characteristics of these instruments.